Geochronology of mafic magmatism and hydrothermal alteration during early stages of South Atlantic opening

40Ar/39Ar laser incremental-heating geochronology of whole-rock fragments and minerals from primitive subalkaline mafic dykes from the Cabo Frio Tectonic Domain, southeastern Brazil, reveals a complex combination of intrusive ages, apparent ages affected by excess argon, and ages that record late-stage hydrothermal alteration. Incremental-heating analysis of encapsulated magmatic amphibole-biotite clusters reveals a minimum intrusive age of 132.83 +/- 0.30 Ma. Sericitized plagioclase crystals show the percolation of K-rich fluids at similar to 106 Ma. Albitized plagioclase phenocrysts suggest the percolation of Na-rich fluids in the 60-30 Ma interval. Hydrothermal alteration events overlap in time with pulses of alkaline magmatism in the extending Brazilian crust, suggesting a link between magmatic activity and generation of fluid circulation cells along the margin and adjacent sedimentary basins. Evidence for pervasive hydrothermal re-setting of the K-Ar system in the Cabo Frio Tectonic Domain dykes suggests that young 40Ar/39Ar geochronology results obtained for mafic dykes here and elsewhere along the continental margins may not record multiple intrusive events, reflecting instead partial or total resetting of the K-Ar system by water-rock interaction during fluid circulation. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Laser incremental-heating geochronology of whole-rock fragments and minerals from primitive subalkaline mafic dykes in the Cabo Frio Tectonic Domain, southeastern Brazil, reveals a complex combination of intrusive ages, apparent ages affected by excess argon, and ages that record late-stage hydrothermal alteration. The study suggests a link between magmatic activity and generation of fluid circulation cells along the margin and adjacent sedimentary basins, showing evidence for pervasive hydrothermal re-setting of the K-Ar system.